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A SIM lock, simlock, network lock or subsidy lock is a technical restriction built into GSM mobile phones by mobile phone manufacturers for use by services providers to restrict the use of these phones to specific countries and/or networks. This is contrary to a regular phone (retrospectively called SIM-free or unlocked) that does not impose any SIM restrictions. Generally, phones can be locked to accept only SIM cards with certain International Mobile Subscriber Identities (IMSIs); IMSIs may be restricted by: * Mobile country code (MCC; e.g., will only work with SIM issued in one country) * Mobile network code (MNC; e.g., AT&T Mobility, T-Mobile, Vodafone, Bell Mobility etc.) * Mobile subscriber identification number (MSIN; i.e., only one SIM can be used with the phone) Additionally, some phones, especially Nokia phones, can lock group IDs which are used in voice group call service. In the United States and other areas, where carriers often offer deeply discounted cell phones in exchange for an exclusive agreement with a carrier, that phone will often be locked so it will not work with another carrier. Most mobile phones can be unlocked to work with any GSM network provider, but the phone may still display the original branding and may not support features of the new carrier; besides the locking, phones may also have firmware installed on them which is specific to the network provider. For example, a Vodafone or Telstra branded phone in Australia will display the relevant logo and may only support features provided by that network (e.g. Vodafone Live!). This firmware is installed by the service provider and is separate from the locking mechanism. Most phones can be ''unbranded'' by reflashing a different firmware version, a procedure recommended for advanced users only. The reason many network providers SIM lock their phones is that they offer phones at a discount to customers in exchange for a contract to pay for the use of the network for a specified time period, usually between one and three years. This business model allows the company to recoup the cost of the phone over the life of the contract. Such discounts are worth up to several hundred U.S. dollars. If the phones were not locked, users might sign a contract with one company, get the discounted phone, then stop paying the monthly bill (thus breaking the contract) and start using the phone on another network or even sell the phone for a profit. SIM locking curbs this by prohibiting change of network (using a new SIM). SIM locking is very common if subsidized phones are sold with prepaid contracts. It is important to note, however, that the technology associated with the phone must be compatible with the technology being used by the network carrier. A GSM cell phone will only work with a GSM carrier and will not work on a CDMA network provider. Likewise, a CDMA cell phone will only work with a CDMA carrier and will not work on a GSM network provider. In some jurisdictions, including China, Israel, and Singapore, SIM locking is illegal , but in other countries, carriers do not provide unlocking codes, or oblige the consumer to pay a fee. Unlocking the phone, however, is almost universally legal.〔(【引用サイトリンク】title=Is iPhone unlocking illegal? )〕 Additionally, it is often legal for carriers to force SIM locks for certain amounts of time, varying by region. == Unlocking technology == A handset can be unlocked by entering a code provided by the network operator. Alternative mechanisms include software running on the handset or a computer attached to the handset, hardware devices that connect to the handset or over-the-air by the carrier. Usually the unlock process is permanent. The code required to remove all locks from a phone is referred to as the ''master code'', ''network code key'', or ''multilock code''. There can also be multiple levels of locks placed on the phone by networks, which block the use of other networks' SIM cards. These are usually referred to as "Network" (NCK) or "Service Provider" (SPCK) locks and can be removed using the corresponding unlock codes, which are unique to each phone depending on its IMEI.〔(【引用サイトリンク】title=Different Types of Unlock Code for Changing Network )〕 Typically, a locked handset will display a message if a restricted SIM is used, requesting the unlock code. On recent phone models running Android software, the phone will display a message saying “SIM network unlock PIN” or “Enter Network Lock Control Key” if network locked. Windows phones will display the message, “This SIM card can only be used on specific networks. Contact your customer service center for the unlock code”. Other handsets may display different messages such as "Enter special code" or "Enter unlocking code," or in some cases the handset will simply display a message stating that it is locked. Once a valid code is entered, the handset will display "Network unlocked” or “Network unlock successful”. The unlock code is verified by the handset and is generated by the manufacturer, typically by an algorithm such as a one way hash or trapdoor function. Sometimes big telecom providers change the original factory unlock codes as an extra layer of security against unlocking services. For various big brands such as Samsung and Motorola there is no algorithm but just a random code generator where the unlock codes are programmed in the phone itself and then saved in a big database managed by the manufacturer. For the other brands where the unlock codes are still based on algorithms those are based on the IMEI number and the MCC code and have been reverse-engineered, stolen or leaked. Some handsets can be unlocked using software that generates an unlock code from an IMEI number and country and operator details using the algorithm specific to the handset. Other manufacturers have taken a more cautious approach, and embed a random number in the handset's firmware that is retained by the network on whose behalf the lock was applied and the manufacturer. These handsets can still be unlocked by online services that have access to either inside people with the manufacturer or with the telecom networks, or they need to be connected to the computer with a cable where specific software will bypass the security and SIM-unlock the phone. Sometimes this is done by advanced calculations to bypass the security the official way and other times using exploits or overwriting parts of the firmware where the lock status is kept, and often even recover a phone that is bricked or completely damaged in the software sense. Most handsets have security measures built into their firmware that protects them from repeated attempts to guess the unlock code. After entering more than a certain number of incorrect codes the phone becomes ''frozen''. This is a state where the phone will display a security message that the phone needs service. Older phones could not be used at all anymore at this point, but modern smartphones often keep on working with the original SIM but need extra work to then unlock them the right way. Depending on the brand this "frozen state" can sometimes be resolved both by a special unfreeze code (Samsung) and cable solution and other brands need special cable solutions. In the worst situations one would need a special so-called "JTAG" hardware solution to fix the phone. This is often needed when there is more damage to the software, often including software damage to the "bootloader". A hardlocked phone is one in which a telecom provider has modified the firmware on the phone to make it impossible to manually enter the unlock codes in any way. The only solution to SIM-unlock such a phone is to change the firmware to a firmware which has not been modified by any telecom provider, a so-called "unbranded firmware". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「SIM lock」の詳細全文を読む スポンサード リンク
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